Excimers
   HOME

TheInfoList



OR:

An excimer (originally short for excited dimer) is a short-lived dimeric or heterodimeric
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
formed from two species, at least one of which has a valence shell completely filled with electrons (for example, noble gases). In this case, formation of molecules is possible only if such atom is in an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
ic
excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to a ...
. Heteronuclear molecules and molecules that have more than two species are also called exciplex molecules (originally short for excited complex). Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the
ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...
. The lifetime of an excimer is very short, on the order of nanoseconds.


Formation and decay

Under the
molecular orbital In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding ...
formalism, a typical ground-state molecule has
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
s in the lowest possible energy levels. According to the Pauli principle, at most two electrons can occupy a given orbital, and if an orbital contains two electrons they must be in opposite spin states. The highest occupied molecular orbital is called the HOMO and the lowest unoccupied molecular orbital is called the LUMO; the energy gap between these two states is known as the HOMO–LUMO gap. If the molecule absorbs light whose energy is equal to this gap, an electron in the HOMO may be excited to the LUMO. This is called the molecule's
excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to a ...
. Excimers are only formed when one of the dimer components is in the excited state. When the excimer returns to the ground state, its components dissociate and often repel each other. The wavelength of an excimer's emission is longer (smaller energy) than that of the excited
monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Mo ...
's emission. An excimer can thus be measured by fluorescent emissions. Because excimer formation is dependent on a
bimolecular In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reactionAtkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014 and is equal to the sum of stoichiometric coeffici ...
interaction, it is promoted by high monomer density. Low-density conditions produce excited monomers that decay to the ground state before they interact with an unexcited monomer to form an excimer.


Usage note

The term ''excimer'' (excited state dimer) is, strictly speaking, limited to cases in which a true dimer is formed; that is, both components of the dimer are the same molecule or atom. The term exciplex refers to the heterodimeric case; however, common usage expands ''excimer'' to cover this situation.


Examples and use

Heterodimeric diatomic complexes involving a noble gas and a
halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...
, such as xenon chloride, are common in the construction of
excimer laser An excimer laser, sometimes more correctly called an exciplex laser, is a form of ultraviolet laser which is commonly used in the production of microelectronic devices, semiconductor based integrated circuits or "chips", eye surgery, and microm ...
s, which are excimers' most common application. These lasers take advantage of the fact that excimer components have attractive interactions in the
excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to a ...
and repulsive interactions in the
ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...
. Emission of excimer molecules is also used as a source of spontaneous ultraviolet light (
excimer lamp An excimer lamp (or excilamp) is a source of ultraviolet light based on spontaneous emission of excimer (exciplex) molecules. Introduction Excimer lamps are quasimonochromatic light sources operating over a wide range of wavelengths in the ult ...
s). The molecule
pyrene Pyrene is a polycyclic aromatic hydrocarbon (PAH) consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This yellow solid is the smallest peri-fused PAH (one where the rings are fused through mor ...
is another canonical example of an excimer that has found applications in biophysics to evaluate the distance between biomolecules. In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
, many reactions occur through an exciplex, for example, those of simple arene compounds with alkenes. The reactions of
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...
and their products depicted are a +2ycloaddition to the ortho product (A), a +3ycloaddition to the meta product (B) and the +4ycloaddition to the para product (C) with simple alkenes such as the isomers of
2-butene But-2-ene is an acyclic alkene with four carbon atoms. It is the simplest alkene exhibiting ''cis''/''trans''-isomerism (also known as (''E''/''Z'')-isomerism); that is, it exists as two geometric isomers ''cis''-but-2-ene ((''Z'')-but-2-ene) and ...
. In these reactions, it is the arene that is excited. As a general rule, the regioselectivity is in favor of the ortho adduct at the expense of the meta adduct when the amount of charge transfer taking place in the exciplex increases.


Generation techniques

It takes a noble gas atom in an excited electronic state to form an excimer molecule such as a noble gas dimer or noble gas halide. Sufficiently high energy (approximately 10 eV) is required to obtain a noble gas atom in the lowest excited electronic state, which provides the formation of an excimer molecule. The most convenient way to excite gases is by an electric discharge. That is why such excimer molecules are generated in a
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...
(see excimer molecule formation).


Fluorescence quenching

Exciplexes provide one of the three dynamic mechanisms by which
fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
is quenched. A regular exciplex has some charge-transfer (CT) character, and in the extreme case there are distinct radical ions with unpaired electrons. If the unpaired electrons can spin-pair to form a covalent bond, then the covalent bonding interaction can lower the energy of the charge transfer state. Strong CT stabilisation has been shown to lead to a
conical intersection In quantum chemistry, a conical intersection of two or more potential energy surfaces is the set of molecular geometry points where the potential energy surfaces are degenerate (intersect) and the non-adiabatic couplings between these states are ...
of this exciplex state with the ground state in a balance of steric effects, electrostatic interactions, stacking interactions, and relative conformations that can determine the formation and accessibility of bonded exciplexes. As an exception to the conventional radical ion pair model, this mode of covalent bond formation is of interest to photochemistry research, as well as the many biological fields using fluorescence spectroscopy techniques. Evidence for the bonded exciplex intermediate has been given in studies of steric and Coulombic effects on the quenching rate constants and from extensive density functional theory computations that show a curve crossing between the ground state and the low-energy bonded exciplex state.


See also

* * * * *


References

{{Excimer lasers Photochemistry